If now the compound is again dissolved or heated, the emanation escapes. Since the active deposit is not volatile and is insoluble in water, it is not removed by the process of solution or heating. Since, however, the parent matter is removed, the activity due to the active deposit will immediately begin to decay, and in the course of a few hours will have almost disappeared. The activity of the radium measured by the α rays is then found to be about 25 per cent. of its original value. This residual activity of radium, consisting entirely of α rays, is non-separable, and has not been further diminished by chemical or physical means. Rutherford and Soddy[314] examined the effect of aspiration for long intervals through a radium chloride solution. After the first few hours the activity was found to be reduced to 25 per cent., and further aspiration for three weeks did not produce any further diminution. The radium was then evaporated to dryness, and the rise of its activity with time determined. The results are shown in the following table. The final activity in the second column is taken as one hundred. In column 3 is given the percentage proportion of the activity recovered. Time in Activity Percentage days Activity recovered 0 25·0 0 0·70 33·7 11·7 1·77 42·7 23·7 4·75 68·5 58·0 7·83 83·5 78·0 16·0 96·0 95·0 21·0 100·0 100·0 The results are shown graphically in Fig. 85. The decay curve of the radium emanation is shown in the same figure. The curve of recovery of the lost activity of radium is thus analogous to the curves of recovery of uranium and thorium which have been freed from the active products Ur X and Th X respectively. The intensity _I_{t}_ of the recovered activity at any time is given by $$ \frac {I_t} {I₀} = 1 − e^{–λt} $$, where _I₀_ is the final value, and λ is the radio-active constant of the emanation. The decay and recovery curves are complementary to one another. [Illustration: Fig. 85.] Knowing the rate of decay of activity of the radium emanation, the recovery curve of the activity of radium can thus at once be deduced, provided all of the emanation formed is occluded in the radium compound.